Assuming you want to send out printed hardcopies for an invoice and you setup the number of copies to 5 in the Customer. If you print, you get 5 copies. If you now also want for informational purposes email the invoice as well - you will get a PDF with 5 copies. Which makes no sense for PDF.
If it would be possible to either setup a alternative number of copies or even set a checkmark in the document sending profile to ignore the number of copies for PDF documents, that would save manual work.